X-ray tube having a rotary anode

ABSTRACT

An X-ray tube having a rotary anode comprising a disc-shaped anode and a substantially cylindrical rotor, wherein the anoderotor assembly is made integrally of a single piece of graphite, whose cylindrical rotor part is provided with a ferromagnetic hollow cylinder whereupon acts the rotating electromagnetic field produced by the stator.

United States Patent [19] in 3,710,162 Bougle 1 Jan. 9, 1973 [54] X-RAY TUBE HAVING A ROTARY [56] References Cited ANODE UNITED STATES PATENTS [75] Inventor: Francoise Bougle, Paris, France 2,336,271 12/1943 Machlett et al. ..313/60 [73] Assignee: Complgnle Generale dc Radlologle,

Paris, France Primary Examiner-Roy Lake Assistant Examiner-Darwin R. Hostetter [22] Filed: Feb. 19, 1971 Attorney-Cushman, Darby & Cushman [21] Appl. No.: 116,879 [57] ABSTRACT I An X-ray tube having a rotary anode comprising a Foreign Appliclfioll Priority Data disc-shaped anode and a substantially cylindrical ro- Feb. 27, 1970 France ..7007192 wherein the assembly is made s v I I tegrally of a single piece of graphite, whose cylindrical [52] U.S.Cl ..313/60, 313/330 rotor part is provided with a ferromagnetic hollow [51] Int. Cl. ..H0lj 35/10 cylinder whereupon acts the rotating electromagnetic [58] Field of Search ..313/60, 330 field produced'by the stator.

10 Claims, 3 Drawing Figures X-RAY TUBE HAVING A ROTARY ANODE The present invention relates to rotary anodes for X-v ray tubes and relates, more particularly, to anodes made substantially of graphite.

The applicant in his U.S. Pat. No. 3,539,859 has described an X-ray tube having a rotary anode comprising a graphite disc attached to a metal rotor, for example'a copper rotor. A device of this kind ensures an improvement, in relation to the prior art, in the dissipation of the heat produced by the electron bombardment of the focal track which is made of a refractory metal of high atomic number such as tungsten, rhenium or an alloy thereof.

Devices of this kind, with a copper rotor and especially where the whole anode is made of a refractory material, have limitations as far as the dissipation of the heat supplied to the anode is concerned.

It has long been known to provide an improvement in this direction by coating the copper with a layer of black oxide (Cr, Ni but this measure alone is insufficient to achieve the dissipation of a large amount of heat when supplied to the anode in a very short time, this especially being the case when repeated X-ray pictures are taken as in vascular examinations, for example.

The device in accordance with the invention enables this drawback to be overcome by achieving better heat dissipation and, moreover, allows a lighter rotor-anode assembly design so that the starting time of the rotation is shortened.

According to the invention, there is provided an X- ray tube having a rotary anode, said anode comprising a first disc-shaped portion and a second portion in the shape of a hollow cylinder, said disc being covered, at least over an area corresponding to the focal track of the tube, with a layer of refractory material generating X-rays under electron bombardment, characterized in that said two anode portions are formed integrally from a single piece graphite having a very fine structure said second cylinder-shaped portion being provided, over at least part of its internal periphery, with a hollow cylinder of a ferromagnetic material, constituting the rotor on which the rotary stator field acts.

This X-ray generating refractory layer may be made of a metal having a high atomic number, as for example, tungsten, rhenium, tantalum, hafnium etc, or ofalloys or of a superimposition of layers of two or more of these metals.

Alternatively the focal track may be covered by a refractory metal carbide layer the melting point of which is considerably higher than that of the metal, resulting in the possibility of increasing the amount of useful emitted radiation by raising the admissible focal temperature. The increase of the useful maximum temperature in this case is such that it more than compensates for the lower specific X-ray yield of this metal carbide which has a lower equivalent atomic number than the metal itself.

A further object of the present invention consists in providing one or more recesses in the graphite and depositing an absorbent getter there.

The invention will be better understood and other of its features rendered apparent from a consideration of the ensuing description and the accompanying drawings, given here by way of example, wherein FIG. 1 represents a sectional view of a part of an X- ray tube in accordance with the invention FIG. 2 shows an anode-rotor assembly according to an embodiment of the invention FIG. 3 shows another embodiment of an anode rotor assembly according to the invention.

In FIG. 1, the reference 1 indicates part of the glass envelope of the X-ray tube. A cylindrical tube-shaped part 2 made of a metal or alloy having practically the same coefficient of expansion as glass enables the envelope l to be welded to a first part 3 made of a nonferrous metal and designed to carry the rotor. A shaft 4 carrying the rotor is connected to the first part 3 by means of two ball bearings 5 and 6. The top end of the shaft 4 is fixed to a second part 7 which terminates in a disc to which the anode-rotor assembly 8 is attached. The anode-rotor assembly 8 is manufactured integrally from one piece of the fine-grain, high density graphite defined in the aforementioned patent specification. A graphite of this kind is obtained by compression and heating, giving rise to pseudo-vitrified material with high mechanical strength, which is also chosen to have a coefficient of expansion close to that of the X-ray emissive metal layer to be deposited upon the focal track.

Said assembly 8 comprises a first disc-shaped portion 9 having an oblique face, which carries the layer 10 generating the X-rays deposited on the focal track and a second portion 11 substantially in the form of a hollow cylinder.

The second portion 11 of the rotor 8 carries a hollow cylinder 12 of a ferromagnetic material force-fitted into its interior, this cylinder constituting the rotor proper upon which the rotary magnetic field generated by a conventional stator (not shown in FIG. 1 acts.

In one of the embodiments of the invention, the disc constituting the first portion 9 comprises at its center a cylindrical hollow 13 which lightens it and through the base of which the rotor 8 is attached to its shaft 4. The hollow 13 will advantageously contain either recesses 14 preferably disposed symmetrically about the rotor axis, or one or more annular grooves. Said recesses 14 or said grooves are filled with a material having absorbent properties, i.e. a getter, such as titanium, zirconium or tantalum. It is also possible to place additional getter pills elsewhere outside the zone of electron bombardment.

It will be observed here that because of the increased dissipation of the heat generated by electron bombardment, this kind of graphite rotor-anode assembly makes it possible to use X-ray emissive layers made of carbides of certain refractory metals having a high atomic number, like Hafnium or Titanium, so that the focal temperatureof the track can be raised and the radiated power can be thus increased.

As far as the rotor (i.e. the cylinder-shaped portion 11 and ferromagnetic cylinder 12) is started a strong eddy current is induced therein by the stator windings.

As the resistivity of the graphite is relatively high (about I500 microohms. cm), the starting of the rotor is relatively slow.

This disadvantage has been overcome by providing a low resistance path to the induced current.

FIGS. 2 and 3 show two embodiments of such an arrangement.

In FIG. 2, the anode-rotor assembly 8 is provided with a second hollow cylinder or cylindrical layer 15 made of a good conductor like copper or silver, located between the outer faceof the ferromagnetic cylinder 12 and the inner face of its cylindrical portion 11. This metal cylinder 15 will provide a low resistance path for the starting current, copper for instance, having a resistivity of about 1.7 microohms. cm. Cylinder 15 can be made from copper, silver or aluminum sheet or tubing, however the most advantageous way of obtaining it is by depositing the metal in a layer on the ferromagnetic cylinder 12 or on the inner or outer faces of the cylindrical rotor portion 11. This layer can be deposited by any known method as for example electrolytically, pyrolytically (deposited in gaseous phase) or by means of a plasma torch.

The preferred embodiment, shown in FIG. 3, comprises a layer of pyrolytic graphite l6 deposited upon the outer face of the cylindrical graphite portion 11 of the assembly 8. Such a layer is obtained by inserting the cylindrical portion 11 heated to a temperature lying between 1500' 2800C into a hydrocarbon atmosphere (containing methane, for example) having a predetermined pressure.

Such a pyrolytic graphite layer is strongly anisotropic as far as its mechanical, electrical and thermal properties are concerned. Thus its electrical resistivity in the directions perpendicular to that of its growth is much lower (comparable to that of copper) than in its growth direction.

Such a layer can be made even more conductive by appropriate heat treatment, such that it becomes a pseudo-monocrystal of carbon. In this case its conductivity can become up to five times that of copper.

The main advantage of such a pyrolytic graphite layer resides in its low specific weight, thus providing the rotor with a conducting layer without adding appreciably to its weight.

Of course, the invention is not limited to the embodiments described and shown which were given solely by way of example.

What is claimed is 1. An X-ray tube having a rotary anode, said anode comprising a first disc-shaped portion and a second portion in the shape of a hollow cylinder, said disc being covered, at least over an area corresponding to the focal track of the tube, with a layer of refractory material generating X-rays under electron bombardment, characterized in that said two anode portions are formed integrally from a single piece graphite having a very fine structure said second cylinder-shaped portion being provided, over at least part of its internal periphery, with a hollow cylinder of a ferromagnetic material, constituting the rotor on which the rotary stator-field acts.

2. X-ray tube as claimed in claim 1, wherein said anode-rotor assembly further comprises a further hollow cylinder or cylindrical layer coaxial with both said second cylindrical portion and said ferromagnetic cylinder and made of a material which is a good electrical conductor at least in direction parallel to its surface.

3. X-ray tube as claimed in claim 2, wherein said further cylinder is metallic.

4. X-ray tube as claimed in claim 3, wherein said metal cylinder or layer is placed in the vicinity of one of the faces of said secondcylinder-shaped portion.

5. X-ray tube as claimed in claim 4, wherein said metal cylinder is made up by a layer of metal deposited upon said second portion.

6. X-ray tube as claimed in claim 2, wherein said further cylinder is built up by a layer of pyrolytical graphite deposited on at least a part of the outer face of said second cylinder-shaped portion.

7. X-ray tube as claimed in claim 6, wherein said pyrolytical graphite layer is heat treated so as to provide pseudo-monocrystalline structure.

8. X-ray tube as claimed in claim 1, wherein said X- ray generating material is a refractory metal.

9. X-ray tube as claimed in claim 1, wherein said X- ray generating material is a carbide of a refractory metal. I

10. X-ray tube as claimed in claim 1, wherein said disc-shaped anode portion comprises one or more recesses or grooves formed in a part thereof not subjected to the electron bombardment, and filled with material having, after its evaporation, gas absorptive properties, in other words a getter. 

1. An X-ray tube having a rotary anode, said anode comprising a first disc-shaped portion and a second portion in the shape of a hollow cylinder, said disc being covered, at least over an area corresponding to the focal track of the tube, with a layer of refractory material generating X-rays under electron bombardment, characterized in that said two anode portions are formed integrally from a single piece graphite having a very fine structure ; said second cylinder-shaped portion being provided, over at least part of its internal periphery, with a hollow cylinder of a ferromagnetic material, constituting the rotor on which the rotary stator field acts.
 2. X-ray tube as claimed in claim 1, wherein said anode-rotor assembly further comprises a further hollow cylinder or cylindrical layer coaxial with both said second cylindrical portion and said ferromagnetic cylinder and made of a material which is a good electrical conductor at least in direction parallel to its surface.
 3. X-ray tube as claimed in claim 2, wherein said further cylinder is metallic.
 4. X-ray tube as claimed in claim 3, wherein said metal cylinder or layer is placed in the vicinity of one of the faces of said second cylinder-shaped portion.
 5. X-ray tube as claimed in claim 4, wherein said metal cylinder is made up by a layer of metal deposited upon said second portion.
 6. X-ray tube as claimed in claim 2, wherein said further cylinder is built up by a layer of pyrolytical graphite deposited on at least a part of the outer face of said second cylinder-shaped portion.
 7. X-ray tube as claimed in claim 6, wherein said pyrolytical graphite layer is heat treated so as to provide pseudo-monocrystalline structure.
 8. X-ray tube as claimed in claim 1, wherein said X-ray generating material is a refractory metal.
 9. X-ray tube as claimed in claim 1, wherein said X-ray generating material is a carbide of a refractory metal.
 10. X-ray tube as claimed in claim 1, wherein said disc-shaped anode portion comprises one or more recesses or grooves formed in a part thereof not subjected to the electron bombardment, and filled with material having, after its evaporation, gas absorptive properties, in other words a getter. 